The goal of the proposal is to determine whether DNA DSBR are killing neurons. DNA repair bas been linked directly to Huntington Disease (HD) onset by genome-wide association studies (GWAS) in patients. CAG expansion occurs in the process of repairing oxidative DNA damage, and its role for DNA repair was well established in mouse models. During the past funding cycle, however, we have discovered that mhtt suppresses DNA repair in brain cells in a cell-type and region-specific manner in the brains of HdhQ(150/150) animals. This had modest effect on CAG expansion, but at the same time, resulted in the accumulation of DNA double strand breaks (DSBs) that had gone undetected. DSBs are the most toxic of lesions, and if not repaired lead to cell death. This raised the issue as to whether the unrepaired and previously unrecognized DSBs were responsible for killing neurons. We have developed a new approach to defining DNA repair alterations in brain cells, called repair fingerprinting. In Aim 1, we will use ?repair fingerprinting? to determine whether DNA double strand breaks are formed or repaired differently in astrocytes and neurons, and what pathways are involved. Repair fingerprinting is an integrated three-arm approach to identify the landscape of DNA repair and extract which pathways and machinery are responsible for the DSBR in brain cells. In Aim 2, we will determine whether cryptic DNA double strand breaks are the primary driver of neuronal death in HdhQ(150/150) mice in vivo.